critique my setup -- weak link? best way to improve?

If the speakers *must* stay on the credenza

I pretty much ignored this part. If you want good sound don't let the furniture determine it, proper stands and placement are literally the easiest and cheapest tweaks of the system and the best bang for the buck. 

If you are forced and willing to compromise with the placement, you are probably not crazy about achieving the best sound. And nothing wrong with that....

Based on your post focusing on your analog needs If you don’t already have the speakers isolated from your sideboard look at Isoacoustics mini pucks at a minimum. Also a standalone phono preamp from the likes of Schiit or others within your budget and an updated cartridge at least in the $200-300 range or more.

An upgraded Preamplifier more in line with your Poweramp would help your analog and digital quality.

Re Seeker, nearly all preamplifiers ever made operate in Class A, and many/ most modern tube Preamplifiers can drive a 10K ohm load easily (which is to say their output Z is 1K ohm or less). I agree those are parameters that need to be checked out before you buy a tube preamp to drive an SS amplifier. You want input Z to be 10X or more higher than output Z.

There are some well proven bookshelf   style speakers that are affordable and you appreciate a much smoother sound after their broken in after 60 hours or so. Those speakers are the Wharfedale Lintons. They are usually on lower price around Xmas /black Friday specials.

Speakers would be the biggest bang for your buck improvement IF you can afford 1500-3k on new speakers.  I am personally a huge fan of Tekton Design Impact Monitors and Double Impact, but any of their designs with the seven tweeter array I'm sure is solid.  Expect to wait weeks though and have trouble getting through to them at times.  Although, they have some models available for sale right now on their specials section.

Otherwise KEF makes solid stuff if you don't like playing loud as I think their low end gets heavily distorted at higher volume typically at lower price points.

But yes, without knowing your budget, it's hard to give you advice.  Speakers are by far going to give you the biggest change in your system though.  But I'd say you'd have to spend $1500+ to get more than an incremental change.  I'd go as far to say that with some $2000+ speakers such as the tektons, it would absolutely change your system into a different animal.

Wish you the best of luck in your system journey!